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| author | Michał Muskała <[email protected]> | 2018-04-27 12:40:07 +0200 |
|---|---|---|
| committer | Michał Muskała <[email protected]> | 2018-04-29 17:03:43 +0200 |
| commit | 5d0874a8f9fd617308d9024783db1e4e24268184 (patch) | |
| tree | ac61f2f0f8d7f7483af9e17df44b49aeb049e695 /erts/preloaded | |
| parent | d423a7af502227afcdcf7d2a1efecded85ea95fb (diff) | |
| download | otp-5d0874a8f9fd617308d9024783db1e4e24268184.tar.gz otp-5d0874a8f9fd617308d9024783db1e4e24268184.tar.bz2 otp-5d0874a8f9fd617308d9024783db1e4e24268184.zip | |
Introduce is_map_key/2 guard BIF
This complements the `map_get/2` guard BIF introduced in #1784.
Rationale.
`map_get/2` allows accessing map fields in guards, but it might be
problematic in more complex guard expressions, for example:
foo(X) when map_get(a, X) =:= 1 or is_list(X) -> ...
The `is_list/1` part of the guard could never succeed since the
`map_get/2` guard would fail the whole guard expression. In this
situation, this could be solved by using `;` instead of `or` to separate
the guards, but it is not possible in every case.
To solve this situation, this PR proposes a `is_map_key/2` guard that
allows to check if a map has key inside a guard before trying to access
that key. When combined with `is_map/1` this allows to construct a
purely boolean guard expression testing a value of a key in a map.
Implementation.
Given the use case motivating the introduction of this function, the PR
contains compiler optimisations that produce optimial code for the
following guard expression:
foo(X) when is_map(X) and is_map_key(a, X) and map_get(a, X) =:= 1 -> ok;
foo(_) -> error.
Given all three tests share the failure label, the `is_map_key/2` and
`is_map/2` tests are optimised away.
As with `map_get/2` the `is_map_key/2` BIF is allowed in match specs.
Diffstat (limited to 'erts/preloaded')
| -rw-r--r-- | erts/preloaded/src/erlang.erl | 9 |
1 files changed, 8 insertions, 1 deletions
diff --git a/erts/preloaded/src/erlang.erl b/erts/preloaded/src/erlang.erl index 53e90a4f2d..5fcad25c6d 100644 --- a/erts/preloaded/src/erlang.erl +++ b/erts/preloaded/src/erlang.erl @@ -135,7 +135,7 @@ -export([insert_element/3]). -export([integer_to_binary/1, integer_to_list/1]). -export([iolist_size/1, iolist_to_binary/1, iolist_to_iovec/1]). --export([is_alive/0, is_builtin/3, is_process_alive/1, length/1, link/1]). +-export([is_alive/0, is_builtin/3, is_map_key/2, is_process_alive/1, length/1, link/1]). -export([list_to_atom/1, list_to_binary/1]). -export([list_to_bitstring/1, list_to_existing_atom/1, list_to_float/1]). -export([list_to_integer/1, list_to_integer/2]). @@ -1121,6 +1121,13 @@ is_alive() -> is_builtin(_Module, _Function, _Arity) -> erlang:nif_error(undefined). +%% Shadowed by erl_bif_types: erlang:is_map_key/2 +-spec is_map_key(Key, Map) -> boolean() when + Key :: term(), + Map :: map(). +is_map_key(_,_) -> + erlang:nif_error(undef). + %% is_process_alive/1 -spec is_process_alive(Pid) -> boolean() when Pid :: pid(). |